A major can be extremely disastrous in a plant, if there are bulk fuel, flammable and combustible liquid storage tanks in it.
The primary purpose to installing a deluge water spray system around such tanks is to limit the heat transfer to such bulk fuel tanks from neighbouring fires, prevent tank damage and tank collapse due to such fires. It also gives more time so that emergency fire services can act swiftly.
Medium Velocity (MV) spray nozzles are commonly used in fixed water spray deluge systems. The selection of the proper MV spray nozzle is made considering the water spray coverage area and density required for the hazard being protected. The system is designed considering water demand for simultaneous operation of all MV-nozzles installed around the tank. As per NFPA 15, water spray applied to vessel surfaces should not be less than 0.25 gpm/ft2 which is 10.2 lpm/m2 of the exposed surface area.
The horizontal distance between the MV-nozzles should be such that their spray patterns overlap each other, and the vertical distance should not be more than 12 feet. Cooling by water run-down is expected in such cases. Minimum operating pressure of any MV-nozzle protecting such hazards should be 20 PSI (1.4 bar).
Medium Velocity water spray nozzle has an external deflector, which discharges water in a directional cone shaped pattern of small droplet size. The water is uniformly distributed over the surface to be protected. The Nozzles are effectively designed to apply water to exposed vertical, horizontal, curved, and irregular shaped surfaces to allow cooling to prevent excessive absorption of heat from external fire and avoid structural damage or spread of fire.
MV-nozzle placement and direction should such that water spray covers exposed surfaces. Dry areas occurring because of incomplete nozzle coverage can also result in formation of few hot spots which could be a weak point for fires to occur.
MV-Nozzle offers required water spray and coverage by selection of k-factor and spray angle. Water discharge patters from all spray nozzles are observed to ensure that the coverage is properly achieved by nozzle positioning and discharge patterns. The water supply to the MV-Nozzle system is controlled by a deluge valve, which shall operate via electrically actuated solenoid valve, or via wet-pilot/dry-pilot detection or a combination of systems. While response time from detection of fire to activation of the spray system deluge valve is considered maximum 40 seconds.
Reverse Action MV-Nozzles are designed specifically to protect tank tops and are installed in upright position for wider-coverage through their hemi-spherical spray pattern.
Reference:
NFPA 15, Standard for Water Spray Fire Systems for Fire Protection